The advent of a new round of stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after-treatment technologies are currently being developed that will treat NOx under these conditions. One of these technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR).
Ammonia is difficult to handle in its pure form in the automotive environment. Therefore, it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea solution (CO (NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea solution is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2). The ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
The delivery of the AUS-32 solution to the exhaust involves precise metering of the fluid and proper preparation of the fluid to facilitate the later mixing of the ammonia in the exhaust stream.
Robert Bosch and Purem each have SCR systems in limited volume production for the heavy-duty diesel engine sector. The urea solution delivery function in these systems involves a physical separation of the critical metering and spray preparation functions.
In the case of the Purem system, the metering control is carried out by a Siemens Deka IV injector mounted in a control block. The metered fluid is transported via a tube to the exhaust. After the metering valve, the fluid is also exposed to compressed air to aid with atomization which will ensure subsequent good mixing with the exhaust gas. The pressurized mixture is then injected into the exhaust.
The Bosch production system also comprises an air-assisted solution with separation of the metering and spray preparation functions. Bosch has also disclosed a system concept that does not use air.
Since air compression is not expected to be available on many future applications of the SCR technology, there is a need to have delivery of the AUS-32 without air-assistance while using a solenoid injector.
There is also a need to provide a thermal barrier between the injector of an RDU and an exhaust pipe that defines a portion of the exhaust gas flow path.
The freezing point of AdBlue is −11 C. An alternative reductant carrier under development, known commercially as Denoxium, has a freezing point of −30 C. In the case of both fluids, it is expected that system freezing will occur in cold climates. A problem then arises of being able to meter fluid sufficiently quickly to the exhaust system upon startup of the engine, especially if fluid has been resident in the RDU and has frozen. In conventional systems, fluid is evacuated from the system and the RDU at engine shutdown to avoid localized freezing of the fluid in the injection unit. In certain instances, complete evacuation of the RDU may require ingress of exhaust gas through an injecting unit. This procedure could cause damage to the injecting unit with the introduction of contaminants.
Thus, there is also a need to prevent freezing of urea solution in an RDU and thereby eliminate the need to evacuate the fluid from the system.
In addition, AdBlue has a boiling point of 104 C at atmospheric pressure. Under certain system configurations, for example when the reductant injection location is downstream of a diesel particulate filter undergoing a regeneration event, and fluid flow through the injector has been stopped, the fluid temperatures in the RDU can exceed this boiling temperature. If the fluid boils under these conditions and heating continues, thermolysis of the urea solution can occur leading subsequently to the creation of deposit forming compounds such as biuret and melamines. These deposits can lead to injector malfunction and should be avoided.
Thus, there is a need to ensure that boiling is minimized inside the fluid supply system and the RDU during periods of extreme heating, specifically during a diesel particulate filter regeneration event.